The long-term objective of this proposal is to understand the mechanisms involved in the digestion and absorption of phospholipids in the intestine. The average Western adult ingests about 4-8 g of phospholipid per day and another 7-21 g per day are secreted into the intestinal lumen via the bile. These phospholipids, and the lysophospholipids and fatty acids produced by pancreatic phospholipase A2 digestion, form mixed bilayer vesicles and mixed micelles in the intestinal lumen. These aggregates are necessary to solubilize other intestinal lipids and digestion products (e.g. cholesterol, monoglycerides, diglycerides, and fat-soluble vitamins and to provide a water-soluble vehicle for transport to the brush border membrane surface. Once absorbed by the mucosal cells, a portion of the lysophospholipids are reacylated to provide the phospholipid surface coat for the formation of chylomicrons and very low density lipoproteins which carry the bulk of the absorbed cholesterol, triglycerides, and other lipids into the circulation. Thus, phospholipid digestion and absorption are integrally related to the intestinal digestion and absorption of most lipids. The goal of this work is to answer three specific questions related to the absorption of phospholipids in the mammalian small intestine: 1) How are phospholipids transferred from mixed lipid vesicles and micelles, in which they are solubilized in the intestinal lumen, to the brush border membrane of the mucosal cells that line the intestinal wall? 2) How do bile salts increase the rate of phospholipid transfer through the water environment? 3) What are the biochemical and functional characteristics of an apparently new lysophospholipase that has recently been purified from intestinal mucosal brush border membranes, where is it produced, and is it structurally related to other known lipases and esterases? Phospholipids and lysophospholipids labeled with the fluorescent probe NBD (7-nitro-2,1,3-benzoxadiazol-4-yl) will be used to study each of these questions. The answers to these questions will lead directly to a better understanding of the fundamental physical and biochemical mechanisms of lipid absorption.